1. Field of the Invention
This invention relates to a fastener and relates particularly to a fastening bolt for use in the concrete.
2. Description of the Related Art
Referring to FIG. 1, a conventional concrete bolt 1 comprises a head 11, a shaft 12 connected to the head 11, a drilling portion 13 disposed at a distal end of the shaft 12 and a plurality of threads 14 spirally disposed on the shaft 12. A screwing operation of the concrete bolt 1 is executed by using an electric drill (not shown) to drill a pilot hole 21 in a concrete wall 2 in advance and then inserting the concrete bolt 1 into the hole 21 for engaging the concrete wall 2. This conventional structure still has problems. For example, the concrete bolt 1 usually has symmetrical threads 14. In other words, an upper flank surface and a lower flank surface of each thread 14 have the same inclined angle relative to a normal line passing an edge where the two surfaces meet, so the flank surfaces has a small gradient because of the symmetrical design. When the bolt 1 is gradually drilled into the hole 21, the flank surfaces of the threads 14 having the small gradient come into contact with the concrete wall 2 and cause some of the threads 14 close to the drilling portion 13 an over-large and incessantly-created frictional resistance. A user may impart more rotating force to overcome the resistance, but it takes time and effort to drill. Further, if more torque is exerted to force the threads 14 to enter the wall 2, the shaft 12 easily swings or enters aslant at the beginning of the drilling action. Thus, the thread structure cannot break the concrete wall 2 effectively and cannot drill vertically. These problems render wall pieces broken by the drilling action unable to travel within the hole 21 easily, so the threads 14 cannot engage the concrete wall 2 firmly after the drilling action. Thus, the fastening effect of the bolt 1 is decreased.